timekeeping: Simplify getboottime()

Subtracting plain nsec values and converting to timespec is simpler
than the whole timespec math. Not really fastpath code, so the
division is not an issue.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NJohn Stultz <john.stultz@linaro.org>

kernel/time/timekeeping.c

@@ -1525,14 +1525,9 @@ void update_wall_time(void)
 void getboottime(struct timespec *ts)
 {
 	struct timekeeper *tk = &tk_core.timekeeper;
-	struct timespec boottime = {
-		.tv_sec = tk->wall_to_monotonic.tv_sec +
-				tk->total_sleep_time.tv_sec,
-		.tv_nsec = tk->wall_to_monotonic.tv_nsec +
-				tk->total_sleep_time.tv_nsec
-	};
-
-	set_normalized_timespec(ts, -boottime.tv_sec, -boottime.tv_nsec);
+	ktime_t t = ktime_sub(tk->offs_real, tk->offs_boot);
+
+	*ts = ktime_to_timespec(t);
 }
 EXPORT_SYMBOL_GPL(getboottime);